There have been increasing demands in recent years for cellulose resin films, such as a cellulose acylate film, as optical materials for protective film of sheet polarizer or optical compensation film of retardation plate in liquid crystal displays.
Processes for forming a cellulose resin film can be classified into two large categories: solution film forming process, and melt film forming process. The solution film forming process is a process in which a dope of cellulose resin in a solvent is cast from a die upon support(s), for example, cooling drum(s) into a film, while the melt film forming process is a process in which a cellulose resin is molten in an extruder and the molten resin is extruded from a die upon support(s), for example, cooling drum(s) into a film. The cellulose resin films formed by these processes are usually stretched in the longitudinal direction (lengthwise direction) and in the transverse direction (widthwise direction) so that it develops in-plane retardation (Re) and across-the-thickness retardation (Rth), and such stretched films are used as a retardation film for liquid crystal display devices to achieve a wider viewing angle in such displays (e.g. Japanese Patent Laid-Open Nos. 2002-311240 and 2003-315551).
Of the above described two film forming processes, the melt film forming process is an environmentally superior process, because it does not need a solvent drying step.